1. Field of the Invention
The present invention relates to a fluid control valve for controlling the flow rate of fluid or the pressure of same.
2. Description of the Prior Art
Of fluid control valves, a hydraulic pressure control valve for controlling hydraulic pressure is heretofore known as for example disclosed in Japanese patent application provisional publication No. 3-121386.
This prior art hydraulic pressure control valve includes a plunger doubling as a valve spool. The plunger is slidable in a stationary body equipped with a solenoid. The stationary body is formed with an inlet side passage and an outlet side passage. The plunger is formed with a cut portion (i.e., control valve portion) for providing or blocking communication between the inlet side passage and the outlet side passage. The plunger is urged in the direction to block the above described communication under the bias of a spring and driven into a position for providing the above described communication by the solenoid and into a position for blocking the above described communication by the outlet side pressure.
However, in the above described prior art hydraulic pressure control valve, in order that the plunger (i.e., valve spool) can slide smoothly, it necessary to form a predetermined clearance between the joining circumferential surfaces of the plunger and an accommodation hole of the stationary body for slidably accommodating therein the plunger, thus causing a problem that the plunger becomes eccentric with the accommodation hole and thus the radial attracting forces of the solenoid becomes unbalanced to allow the plunger to be driven by a transversal force so that the hysteresis in the operation of the hydraulic control valve is enhanced, and that the plunger is liable to catch the accommodation hole due to its inclination and thus incapable of sliding smoothly.
Further, in the prior art hydraulic pressure control valve, the control valve portion is formed in a large diameter circumferential portion of the plunger, thus causing a problem that the amount of leakage fluid is large.
Further, in the prior art hydraulic pressure control valve, the cut portion (i.e., control valve portion) for providing or blocking communication between the inlet side passage and the outlet side passage, is a rectangular groove formed in the circumferential surface of the plunger. Such a rectangular groove can constitute part of a magnetic path during the time of energization of the solenoid and cause an axial attracting force which exerts a bad influence upon the hydraulic pressure control characteristic. In order to avoid this, it becomes necessary to make larger the axial length of the plunger so that the rectangular groove can be formed in the place apart from the magnetic path. For this reason, the following problems are caused.
(1) The axial length of the hydraulic pressure control valve becomes large so that there is a difficulty in making the control valve compact. PA1 (2) The plunger and the stationary body for installation of the plunger are so long and complicated in shape, thus causing a high cost. PA1 (3) The plunger becomes so long that the hysteresis in the operation of the hydraulic pressure control valve is enhanced and the responsibility of same is deteriorated.